Can you explain the sort of sinusoidal path here:
Hmmm, I'm guessing that there is total internal reflection occuring inside the transparent tube. The inner surface appears to be slightly curved. IMO, those are straight lines between the reflection points, and it's not exactly sinusoidal. I'm not certain though.
I think that sums it up. Total internal reflection for the light travelling right to left. The reason we see the green light is that some scattering of the beam is occuring within the tube - otherwise it would be invisible. The scattered light, to reach out eye, has to travel through the tube material and the air, and in doing so, is refracted as it changes media. Since the tube is curved, as it appears to be, the refraction angle changes from the middle of the tube to the top surface, giving the appearance of a sine-wave ( instead of a triangle wave ).
I think you are right, but I think also there must be some kind of coating as the light seems to get trapped into the exterior. Coating clear acrylics is quite common I believe to prevent scratching.
It looks as if it is not a tube but a rod having tapered refractive index, in the same way as an optical fibre. Various modes can be excited by altering the direction of the wave launched into the waveguide by using a laser.
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